Plain English Meaning
A switch or knob in the cockpit that lets the pilot pick which fuel tank the engine drinks from, or shut the fuel off completely.
Full Definition
A cockpit-mounted valve control used by the pilot to choose which fuel tank (or tanks) feeds the engine, and in many aircraft to shut off fuel flow entirely. Typical positions include LEFT, RIGHT, BOTH, and OFF, though exact options vary by aircraft.
Context Anchor
You check the fuel selector during the visual preflight assessment, before engine start, and any time the airplane procedure calls for changing tanks.